Heretofore commercial chrome-alumina fused refractory materials have generally comprised a two-phase crystal structure of chrome-containing corundum solid solution and chrome-alumina spinel solid solution with MgO and iron oxides. The earlier compositions as described in U.S. Pat. No. 2,063,154 emphasized the spinel solid solution. However, the later compositions of U.S. Pat. No. 2,279,260 limited the spinel solid solution by restricting the MgO and FeO contents as well as by small additions of SiO.sub.2, TiO.sub.2, ZrO.sub.2 or B.sub.2 O.sub.3. The latter compositions were the result of a study further reported by H. N. Baumann, Jr. in the Journal of the American Ceramic Society, Vol. 27, No. 11 (1944), pp. 327-329.
Those chrome-alumina fused refractory materials have been commercially employed in forming some structures for containing molten glasses, viz. those in which green coloration imparted by Cr.sub.2 O.sub.3 dissolved from the refractory is not objectionable (e.g. some fibrous or wool glasses and some container glasses). In the English translation of his paper (Paper #20) presented at the Symposium On Refractory Materials for Glass-Melting Furnaces at the Research Laboratories of Pilkington Brothers, Ltd. in England during October 1964, M. Jaupain reported on experiments with several variations of chrome-alumina fused refractories, and he concluded that those with the least magnesia content (and the resultant least spinel phase) exhibited the greatest resistance to corrosion by sheet glass accompanied by negligible coloration of the glass. Nevertheless, commercial chrome-alumina fused refractories made during the past approximately fifteen years have customarily been in accordance with the corrosion and thermal shock resistant, balanced composition specified in U.S. Pat. No. 3,188,219, with about 3.7-7.3 wt.% MgO and about 2.7-4.9 wt.% iron oxide providing a minor spinel matrix as a second phase dispersed among the chrome-containing corundum solid solution phase.
Although the chrome-alumina fused refractories are inherently more resistant to corrosion by many glasses than are the common alumina-zirconia-silica (AZS) fused refractories and the common alumina fused refractories, their higher cost has often militated against their use in the past even where coloration of glass could be tolerated. With the conversion to electric melting and to higher production rates in furnaces producing glasses such as those for glass wool (e.g. thermal insulation for buildings) and glass containers where coloration is not objectionable, increased consumption rates of those other common fused refractories used therein began to cause serious concerns. As a consequence, it became economical to substitute the more corrosion resistant chrome-alumina fused refractories as well as the chrome-modified AZS fused refractory described in U.S. Pat. No. 3,837,870. Additionally, concerns of electrical energy loss through the refractory structure, with rather modest electrical resistivities at operating temperatures, sometimes could be alleviated by special changes in furnace designs, such as are illustrated by U.S. Pat. Nos. 3,806,621, 3,818,112 and 3,967,046, and by British patent specification 1,437,091. But despite these refractory substitutions and design changes, there has continued a real need for a refractory with even better corrosion resistance to the molten glasses and better electrical resistivity to avoid electrical energy losses, along with good thermal shock resistance.
As a preliminary part of our discovery of the present invention, we noted that it is not practically possible to simply eliminate magnesia and/or the spinel solid solution phase from the chrome-alumina fused compositions and thereby obtain increased corrosion resistance. That would result in a single phase mass of substantially elongated and mutually oriented chrome-containing corundum solid solution crystals associated with significant intergranular voids and planes of weakness contributing both poor thermal shock resistance and increased opportunity for corrosion by molten glass. Thus, we observed that the secondary phase is necessary for being able to maintain a finer grained, unoriented, denser crystal structure providing a chrome-alumina fused material with good strength, good thermal shock resistance and as little diminution as possible of corrosion resistance to molten glass contributed by the intrinsic character of the primary corundum solid solution.
The main problem that confronted us was finding a new fused refractory material of a chrome-alumina based composition with some new and/or modified secondary phase or phases that would coact with the corundum solid solution to provide significantly improved corrosion resistance to molten glass vis-a-vis prior chrome-alumina fused refractory materials. A further problem we faced was also finding some compositional modification that would yield notably higher electrical resistivity along with the enhanced corrosion resistance.
Among prior fused refractory materials with a primary corundum phase without chrome, a variety of oxidic secondary phases (besides zirconia in the AZS types) have been employed for various purposes (e.g. see U.S. Pat. Nos. 2,474,544-2,695,849-3,230,101-3,264,123-3,844,803-3,879,210). However, such collective knowledge did not provide any clear definitive guidance to the solution of the problems confronting us.